A standard flow-control assembly has two valves connected in series and operated by respective electromagnets to control flow between a source and a load. Each valve has a spool or slider carrying the core of the solenoid. For maximum resistance to pressure both electromagnets must be electrically energized for flow between the load and the source.
One of the disadvantages of this type of arrangement is in its slow and varying response time. The heavy valve body moves sluggishly, so that there is a lag between application of voltage to the respective coil and the start of movement of the valve body and another lag when the coil is deenergized and the field collapses. This lag is due to the time it takes the magnetic field to build up or decay, the inertia of the valve body, the friction between this body and the valve housing, and numerous other factors. In addition as the valve ages the response time varies due to erosion of the control surfaces, wear of the sliding surfaces, leakage and so on. What is more, the response time also varies with temperature and viscosity of the fluid being controlled, as well as with pressure and other characteristics.
This variable and sluggish response time is therefore impossible to predict accurately, and estimating it is complicated further by the fact that two such valves must taken into account. As a result such a valve assembly cannot accurately dose the fluid it controls.